Precomputed Global Illumination of Isosurfaces

Three dimensional scalar heightfields, also known as volumetric datasets, abound in science and medicine. Viewing the isosurfaces, or level sets, is one of the two main ways to display these datasets, the other being volume visualization. Typically the isosurfaces are rendered on a personal computer (PC) allowing the scientist or doctor analyzing the dataset to interactively change the isovalue, and rotate or zoom the isosurface. Unfortunately, out of necessity due to the PC's video card, current techniques render the isosurfaces with a basic hardware-accelerated lighting model. This lighting model lacks important features such as shadows, and as a result the isosurfaces are more difficult to interpret than if they had been rendered with a physically based lighting model. My thesis is that isosurfaces can be displayed with realistic illumination at interactive rates on a typical PC. I present a method for applying global illumination to interactively created isosurfaces, using a physically based lighting model, with a negligible increase in the time required to render the isosurfaces. The result is convincing shading that is easy to interpret by the human visual system, including features such as soft shadows, inter-reflection, caustics, and color bleeding. This is achieved by solving the rendering equation for all isosurfaces within the volume, storing the solutions in a 3D texture, and then texture mapping the result onto a polygonal approximation of the isosurface. This process is called "heightfield rendering".